Add shared memories (#4187)

* Add shared memories

This change adds the ability to use shared memories in Wasmtime when the
[threads proposal] is enabled. Shared memories are annotated as `shared`
in the WebAssembly syntax, e.g., `(memory 1 1 shared)`, and are
protected from concurrent access during `memory.size` and `memory.grow`.

[threads proposal]: https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md

In order to implement this in Wasmtime, there are two main cases to
cover:
    - a program may simply create a shared memory and possibly export it;
    this means that Wasmtime itself must be able to create shared
    memories
    - a user may create a shared memory externally and pass it in as an
    import during instantiation; this is the case when the program
    contains code like `(import "env" "memory" (memory 1 1
    shared))`--this case is handled by a new Wasmtime API
    type--`SharedMemory`

Because of the first case, this change allows any of the current
memory-creation mechanisms to work as-is. Wasmtime can still create
either static or dynamic memories in either on-demand or pooling modes,
and any of these memories can be considered shared. When shared, the
`Memory` runtime container will lock appropriately during `memory.size`
and `memory.grow` operations; since all memories use this container, it
is an ideal place for implementing the locking once and once only.

The second case is covered by the new `SharedMemory` structure. It uses
the same `Mmap` allocation under the hood as non-shared memories, but
allows the user to perform the allocation externally to Wasmtime and
share the memory across threads (via an `Arc`). The pointer address to
the actual memory is carefully wired through and owned by the
`SharedMemory` structure itself. This means that there are differing
views of where to access the pointer (i.e., `VMMemoryDefinition`): for
owned memories (the default), the `VMMemoryDefinition` is stored
directly by the `VMContext`; in the `SharedMemory` case, however, this
`VMContext` must point to this separate structure.

To ensure that the `VMContext` can always point to the correct
`VMMemoryDefinition`, this change alters the `VMContext` structure.
Since a `SharedMemory` owns its own `VMMemoryDefinition`, the
`defined_memories` table in the `VMContext` becomes a sequence of
pointers--in the shared memory case, they point to the
`VMMemoryDefinition` owned by the `SharedMemory` and in the owned memory
case (i.e., not shared) they point to `VMMemoryDefinition`s stored in a
new table, `owned_memories`.

This change adds an additional indirection (through the `*mut
VMMemoryDefinition` pointer) that could add overhead. Using an imported
memory as a proxy, we measured a 1-3% overhead of this approach on the
`pulldown-cmark` benchmark. To avoid this, Cranelift-generated code will
special-case the owned memory access (i.e., load a pointer directly to
the `owned_memories` entry) for `memory.size` so that only
shared memories (and imported memories, as before) incur the indirection
cost.

* review: remove thread feature check

* review: swap wasmtime-types dependency for existing wasmtime-environ use

* review: remove unused VMMemoryUnion

* review: reword cross-engine error message

* review: improve tests

* review: refactor to separate prevent Memory <-> SharedMemory conversion

* review: into_shared_memory -> as_shared_memory

* review: remove commented out code

* review: limit shared min/max to 32 bits

* review: skip imported memories

* review: imported memories are not owned

* review: remove TODO

* review: document unsafe send + sync

* review: add limiter assertion

* review: remove TODO

* review: improve tests

* review: fix doc test

* fix: fixes based on discussion with Alex

This changes several key parts:
 - adds memory indexes to imports and exports
 - makes `VMMemoryDefinition::current_length` an atomic usize

* review: add `Extern::SharedMemory`

* review: remove TODO

* review: atomically load from VMMemoryDescription in JIT-generated code

* review: add test probing the last available memory slot across threads

* fix: move assertion to new location due to rebase

* fix: doc link

* fix: add TODOs to c-api

* fix: broken doc link

* fix: modify pooling allocator messages in tests

* review: make owned_memory_index panic instead of returning an option

* review: clarify calculation of num_owned_memories

* review: move 'use' to top of file

* review: change '*const [u8]' to '*mut [u8]'

* review: remove TODO

* review: avoid hard-coding memory index

* review: remove 'preallocation' parameter from 'Memory::_new'

* fix: component model memory length

* review: check that shared memory plans are static

* review: ignore growth limits for shared memory

* review: improve atomic store comment

* review: add FIXME for memory growth failure

* review: add comment about absence of bounds-checked 'memory.size'

* review: make 'current_length()' doc comment more precise

* review: more comments related to memory.size non-determinism

* review: make 'vmmemory' unreachable for shared memory

* review: move code around

* review: thread plan through to 'wrap()'

* review: disallow shared memory allocation with the pooling allocator

crates/environ/src/module.rs

@@ -10,7 +10,7 @@ use std::mem;
 use std::ops::Range;
 use wasmtime_types::*;
 
-/// Implemenation styles for WebAssembly linear memory.
+/// Implementation styles for WebAssembly linear memory.
 #[derive(Debug, Clone, Hash, Serialize, Deserialize)]
 pub enum MemoryStyle {
     /// The actual memory can be resized and moved.
@@ -18,7 +18,7 @@ pub enum MemoryStyle {
         /// Extra space to reserve when a memory must be moved due to growth.
         reserve: u64,
     },
-    /// Addresss space is allocated up front.
+    /// Address space is allocated up front.
     Static {
         /// The number of mapped and unmapped pages.
         bound: u64,
@@ -160,7 +160,7 @@ pub enum MemoryInitialization {
     ///   which might reside in a compiled module on disk, available immediately
     ///   in a linear memory's address space.
     ///
-    /// To facilitate the latter fo these techniques the `try_static_init`
+    /// To facilitate the latter of these techniques the `try_static_init`
     /// function below, which creates this variant, takes a host page size
     /// argument which can page-align everything to make mmap-ing possible.
     Static {
@@ -919,6 +919,28 @@ impl Module {
         }
     }
 
+    /// Convert a `DefinedMemoryIndex` into an `OwnedMemoryIndex`. Returns None
+    /// if the index is an imported memory.
+    #[inline]
+    pub fn owned_memory_index(&self, memory: DefinedMemoryIndex) -> OwnedMemoryIndex {
+        assert!(
+            memory.index() < self.memory_plans.len(),
+            "non-shared memory must have an owned index"
+        );
+
+        // Once we know that the memory index is not greater than the number of
+        // plans, we can iterate through the plans up to the memory index and
+        // count how many are not shared (i.e., owned).
+        let owned_memory_index = self
+            .memory_plans
+            .iter()
+            .skip(self.num_imported_memories)
+            .take(memory.index())
+            .filter(|(_, mp)| !mp.memory.shared)
+            .count();
+        OwnedMemoryIndex::new(owned_memory_index)
+    }
+
     /// Test whether the given memory index is for an imported memory.
     #[inline]
     pub fn is_imported_memory(&self, index: MemoryIndex) -> bool {